Semiconductor device and process for producing the same, and lead frame used in said process

ABSTRACT

According to the present invention, as improvement in the adhesion of inner leads with a packaging resin in a resin-sealed semiconductor device is attained by spreading leads on or near the circuit-forming face of a pellet, or on or near the main non-circuit-forming face of the pellet to extend the lengths of the inner leads on or under the pellet.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 529,448, filedMay 29, 1990 (now U.S. Pat. No. 5,126,821), which is a divisional ofapplication Ser. No. 445,942, filed Dec. 8, 1989 (now U.S. Pat. No.4,943,843), which is a continuation of application Ser. No. 240,605,filed Sep. 6, 1988 (now abandoned), which is a continuation ofapplication Ser. No. 845,332, filed Mar. 21, 1986 (now abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to a technique that is especiallyapplicable for providing an electrical connection to a pellet in aresin-sealed semiconductor device.

FIG. 8 is a plan view of a conventional lead frame, which is used in theproduction of a conventional resin-sealed semiconductor device,specifically a resin-sealed type of 64 KSRAM (memory) LSI. In thefigure, the numeral 100 indicates a frame, 101 an outer frame, 102 aninner lead, 103 a tie bar, 104 a tab, and 105 a tab lead.

In the resin-sealed semiconductor device, there is a tendency that thedistance between the side end of a package and the tab, which is apellet mounting part, becomes increasingly narrower with a trend of thepellet size being increased. This is attributed to the fact that thesize of a package for pellets is standardized and, hence, cannot beenlarged despite the increasing size of the pellet.

As a result, it is anticipated that, since this may entail largelylowered adhesion of so-called short leads which are structurally shortlengths of the portions of the leads as external terminals where theleads are embedded within a resin constituting the package, the leadsare liable to easily fall off and peeling is liable to occur between theleads and the resin during bending work of the lead.

The inventors of the present invention have found that this may lead topoor electrical connection, reduction in moisture resistance, etc., toreduce the reliability of the semiconductor device.

Resin-sealed semiconductor devices are described in "IC-Ka JissoGijutsu", pp. 149-150, edited by Nihon Microelectronics Society andpublished by Kogyo Chosakai Publishing Co., Ltd. on Jan. 15, 1980.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a technique whichserves to remarkably improve the adhesion of a resin of a package withthe leads in a resin-sealed semiconductor device, especially in asemiconductor device on which a large semiconductor pellet is mounted.

The aforementioned and other objects and novel features of the inventionwill become apparent from the following description and the accompanyingdrawings.

An outline of a representative embodiment of the invention will now bebriefly described.

Specifically, an improvement in adhesion between inner portions of theleads and a resin constituting a package is achieved by spreading a leadon or near the circuit-forming face of a pellet to be mounted, or on ornear the main non-circuit-forming face of the pellet in a resin-sealedsemiconductor device to enable the inner portion of the lead; i.e., theportion embedded in the resin, to be elongated and thereby provide agreater contact area with the resin.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other objects and novel features of the presentinvention will become apparent from the following description taken withreference to the accompanying drawings in which:

FIG. 1 is a cross-sectional view taken along line I--I in FIG. 2 showinga semiconductor device of Example 1 according to the present invention;

FIG. 2 is a plan view showing the relation of a pellet and the leads ofthe semiconductor device of Example 1;

FIG. 3 is a partial plan view of a lead frame used in the semiconductordevice of Example 1, and shows a state of the frame in the step ofplating;

FIG. 4 is a plan view showing the relation of a pellet and the leads ofa semiconductor device of Example 2 according to the present invention;

FIG. 5 is a partial cross-sectional view taken along line V--V in FIG. 4showing the internal structure of the semiconductor device of Example 2;

FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. 7showing a semiconductor device of Example 3 according to the presentinvention;

FIG. 7 is a plan view showing the relation of a pellet and leads of thesemiconductor device of Example 3; and

FIG. 8 is a plan view of a lead frame used in the production of aconventional resin-sealed semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

FIG. 1 is a cross-sectional view taken along line I-I in FIG. 2 showinga semiconductor device of Example 1 according to the present invention.FIG. 2 is a plan view showing the relation of a pellet and the leads ofthe semiconductor device of Example 1.

The semiconductor device of Example 1 is a so-called resin-sealedsemiconductor device. Specifically, the semiconductor pellet 1 isembedded together with the inner portions of the leads 2, which portionsof the leads 2 serve as external terminals in a resin 4 constituting apackage 3, such as an epoxy resin (hereinafter referred to as "packagingresin"), and the outer portions of the leads 2 outside the package arebended downward near the side ends of the package 3.

In a conventional resin-sealed semiconductor device, a pellet isattached to a tab, which serves as a mounting board having substantiallythe same size as that of the pellet, and bonding pads, which serve asthe electrodes of the pellet, are electrically connected with the innerend portions of the leads disposed at the periphery of the tab via finemetallic wires which serve as bonding wires.

By contrast, in the semiconductor device of Example 1, the leads arespread (i.e., arranged to extend in an elongated manner) on the reverseside of the pellet 1 (namely, the main non-circuit-forming face on whichno semiconductor integrated circuit is formed). The pellet is attachedvia an adhesive 6 to an insulating sheet 5 of a polyimide resin bondedto the leads. In this case, the lead frame has no tab which serves asthe pellet mounting part. The insulating sheet 5 serves to provideelectrical insulation between the leads. In this Example, if there wereno insulating sheet 5, an electrical short circuit would be producedbetween the leads by the conductive pellet 1. In order to avoid theelectrical short circuit, the insulating sheet 5 is disposed between thepellet 1 and the leads.

As shown in FIG. 2, the positional relation of the above-mentionedpellet 1 and the leads 2 is such that the inner portions of the leads 2awhose outer portions are disposed along the sides of the pellet 1 nearwhich no bonding pads 7 are formed in the arrangement (hereinafterreferred to also as "non-bonding pad-forming sides") are spread on thereverse side (main non-circuit-forming face) of the pellet 1, and thatthe tip portions 2b of the leads are spread up to the positionsexceeding the sides of the pellet near which bonding pads 7 are formedin the arrangement. The insulating sheet 5 is bonded onto the leads 2a,and the pellet 1 is attached on the upper face of the insulating sheet 5with the main non-circuit-forming face disposed on the lower side.

In the conventional resin-sealed semiconductor devices, the positionwhere the leads, corresponding to the leads 2a of the invention havingthe inner lead portions spread under the pellet 1, are embedded in thepackaging resin is the region of the packaging resin where the innerlead portions (those portions of the leads which are embedded in thepackaging resin) can merely secure an extremely limited short lengthranging from the side end of the package to the proximity of the tab;namely, the place where the short leads are provided.

The conventional short leads usually cause a problem in that they areliable to fall off from the package because of low tensile strengthattributed to small bonding areas thereof shared with the package resin.Since the bonding areas shared by the short leads and the packagingresin are reduced with an increasing size of the pellet, this problemhas become serious with an increase in the pellet size.

By contrast, since the inner portions of the leads 2a provided in theplaces corresponding to the positions of the above-mentioned short leadsare very long; i.e., these portions are elongated, in the semiconductordevice of Example 1, the bonding areas thereof shared with the packagingresin are large. Thus, the bonding strength between the leads and thepackaging resin can be largely improved. Therefore, peeling in theinterface between the leads and the packaging resin which may occur inbending work of the outer portions of the leads or the like can beeffectively prevented even in a semiconductor device in which a largepellet is used, thus avoiding intrusion of water up to the pellet in thepackaging resin through the peeled portion from the outside. Thus, themoisture resistance of the semiconductor device can be improved.Furthermore, since the insulating sheet 5 is strongly bonded to theleads 2a, the latter have a very high tensile strength.

Moreover, since the leads 2a, formed of a metallic material havinghigher heat conductivity and heat dissipating ability than the packagingresin, are attached to the pellet 1 over a wide range of the facethereof though the attachment made via the insulating sheet 5, the heatgenerated in the pellet in an operating state can be directly dissipatedtoward the outside via the leads. Thus, the semiconductor device of thepresent invention is one having a structure with an excellent heatdissipating ability.

Besides, since the electrical connection of the pellet 1 with the leads2a is achieved by wire-bonding the bonding pad 7a in the pellet 1 withthe tip portions 2b of the leads 2a spread near them and near the sidesof the pellet, the bonding wires 8 can be shortened. Therefore, acontact accident between mutually adjacent bonding wires, between abonding wire and an adjacent lead, or between a bonding wire and thepellet can be prevented. In other words, occurrence of a short circuitaccident can be prevented. Further, the amount of the wires 8 can bedecreased due to the small length of the bonding wires, thus attainingcost reduction.

The semiconductor device of Example 1 is easily prepared by forming alead frame for predetermined shapes of leads, bonding an insulatingsheet 5 in the predetermined portions of the inner portions of theleads, attaching a pellet 1 to the insulating sheet 5 via an adhesive,effecting wire-bonding of the bonding pads 7 of the pellet 1 with thebonding parts of the leads, and following the same assembly step as inthe case of the conventional resin-sealed semiconductor device. In thiscase, the insulating sheet 5 not only serves as an insulator forpreventing a short circuit accident between the leads, but also plays arole of reinforcing the lead frame in respect of the mechanical strengththereof.

The wire-bonding parts of the above-mentioned leads can be formed bydepositing gold according to, for example, the partial plating method.

FIG. 3 is a partial plan view of the lead frame having the insulatingsheet 5 bonded thereto in the portion thereof substantially inside thetie bar 9. The lead frame portions not shown in the figure, for example,the frame portion and the outer lead portion, have shapes similar tothose of a lead frame shown in FIG. 8. In the semiconductor device ofExample 1, since the insulating sheet 5 itself functions as a partialplating mask, use of only a partial plating mask having an opening asshown by the dot mark domain in FIG. 3 makes it possible to effectselective partial plating of a material having excellent wire bondingability, such as gold (Au), in only the tip portions 2b of the leads 2.Therefore, in the case of effecting partial plating, the maskpreparation process is simplified, thus enabling easy formation of thebonding parts.

In FIG. 3 a mask providing spaces only along the short sides of thepellet is shown. However, use of a mask having an opening also providingspaces along the long sides of the pellet parallel to the tie bars 9makes it possible to easily effect partial plating on the leads allaround the insulating sheet 5. In this way, a semiconductor devicehaving pellet whose periphery is provided all along with bonding padscan be easily prepared.

Example 2

FIG. 4 is a plan view showing the relation of a pellet and leads of asemiconductor device of another Example according to the presentinvention.

The semiconductor device of Example 2 is different from that of Example1 in that no insulating sheet 5 is used and that a smaller tab than thepellet 1 is used.

Specifically, in the semiconductor device of Example 2, the pellet 1 isattached, via an adhesive 11 of an insulating material, to the tab andthe inner portions 2b of leads 2a whose outer portions are arrangedalong the non-bonding pad-forming sides of the pellet. Usable adhesivesof insulating materials include polyimide resins, silicone rubbers, andceramics.

In Example 2, since there is no insulating sheet 5, heat can be directlydissipated from the pellet 1. Thus the heat resistance is furtherlowered as compared with that in Example 1 and, hence, the reliabilityis correspondingly higher.

Furthermore, since the tab 10 is attached, the pellet attachmentstrength is also secured.

FIG. 5 is a partial cross-sectional view showing the state of electricalconnection of the pellet 1 with the tip portions of the leads 2 in theview taken along line V--V in FIG. 4. A recess 2c is formed in the tipportion 2b of a lead 2a. In bonding of the pellet 1 with the adhesive11, since the adhesive 11 may flow out to contaminate the surface of thebonding part 12, bonding of the bonding pad 7 of the pellet 1 and thebonding part 12 with a wire 8 cannot sometimes be effected. Theabove-mentioned recess 2c is provided as a dam serving to intercept theflow of the adhesive 11 for avoiding the occurrence of failure of thebonding.

Example 3

FIG. 6 is a cross-sectional view of a semiconductor device of stillanother Example according to the present invention. FIG. 7 is a planview showing the relation of a pellet and leads in the above-mentionedsemiconductor device.

The semiconductor device of Example 3 is different from those ofExamples 1 and 2 in that the inner portions of the leads are spread onthe circuit-forming face of the pellet.

Specifically, as shown in FIG. 6, the pellet 1 is attached, via anadhesive 6, onto the circuit-forming face of the pellet via aninsulating sheet 5 of a polyimide resin bonded onto the reverse face ofthe inner portions of the leads. In this case, the insulating sheet 5functions as an insulator for preventing a short circuit accidentbetween the leads. Besides, the insulating sheet 5 plays a role ofreinforcing the leads in respect of the mechanical strength thereof.

As shown in FIG. 7, the insulating sheet 5 has such a size that it doesnot cover the bonding pads of the pellet 1 bonded thereto. The innerportions of the leads 2a having outer portions arranged along thenon-bonding pad-forming sides of the pellet are spread on the upper faceof the insulating sheet. The inner portions of the leads 2a are bondedto the above-mentioned insulating sheet 5a with the tip portions thereofpositioned short of the bonding pads.

Since the semiconductor device of Example 3 has the inner portions ofthe leads bonded on the side of the circuit-forming face of the pellet1, it is superior in heat dissipating ability to the device of Example1.

The polyimide resin of the insulating sheet 5 serves to prevent asemiconductor element from malfunctioning when the semiconductor deviceis irradiated with α rays from outside. Namely, the insulating sheet 5plays a role of blocking the α rays intruding the device from outside tointercept irradiation of the semiconductor element with the α rays.Since the insulating sheet covers the circuit-forming face, animprovement in reliability against α rays is attained, too.

In a semiconductor device having an insulating film applied as thepassivation film protecting the electric wiring and circuit portions onthe circuit-forming face of the pellet 1, the insulating sheet forpreventing short circuit between the leads may be unnecessary, or notonly an insulating material but also a conductive material can be usedas the adhesive for bonding the pellet to the leads and, if necessary,to the tab.

In Example 3, the tip portions 2b of the leads 2a are disposed in theinner positions short of the bonding pads 7, the positional relationtherebetween is inversed in comparison with that in Example 1. Thus, thebonding direction is inversed, too. However, the bonding distance issubstantially the same as in Example 1.

The effects of the present invention are as follows.

(1) In the resin-sealed semiconductor device, the spread of innerportions of leads on or near the circuit-forming face of a pelletmounted in the device, or on or near the main non-circuit-forming faceof the pellet can greatly improve the adhesion of the inner portions ofthe leads with the packaging resin. Therefore, even where a large pelletis mounted, the leads can be prevented from falling off from thepackaging resin.

In the present invention, at least one of the inner portions of theleads is spread on or under the pellet. As a result, the adhesion of theinner portions with the packaging resin can be greatly improved.Therefore, even where a large pellet is mounted, leads can be preventedfrom falling off from the packaging resin.

(2) For the same reason as mentioned in (1) above, occurrence of peelingin the bonding face between the leads and the packaging resin can beprevented in the bending work of the outer portions of the leads.

(3) For the reasons as mentioned in (1) and (2) above, even in the caseof a semiconductor device having a small package and a large pelletmounted therein, there can be provided a highly reliable semiconductordevice excellent in moisture resistance.

(4) When the inner portions of the leads are attached onto the mainnon-circuit-forming face of the pellet, heat generated in an operatingstate can be effectively dissipated through the leads toward outside.

(5) When an insulating sheet is provided between the pellet and theinner leads in the device as mentioned in (4) above, the attachmentstrength of the pellet can be improved.

(6) The structure as mentioned in (5) above can be easily formed bypreparing a lead frame having an insulating sheet bonded thereto in apredetermined portion thereof and attaching a pellet onto the insulatingsheet.

(7) Since the leads can be reinforced by bonding the insulating sheet tothe lead frame in the predetermined portion thereof, the lead frame canbe easily handled even if it includes a large number of fine leads.

(8) A combination of the lead frame having the insulating sheet bondedthereto in the predetermined portion thereof and a partial plating maskhaving an opening with a size providing a space(s) all or partiallyaround the insulating sheet enables the portions of the leadscorresponding to the above-mentioned space(s) to be easily subjected topartial plating since the insulating sheet functions as a mask ofpartial plating, too.

(9) Attachment of the inner portions of the leads onto thecircuit-forming face of the pellet enables heat generated in the circuitin an operating state to be more directly dissipated through the leads.

(10) Provision of the insulating sheet, which serves to interceptirradiation of semiconductor elements with α rays between the innerleads and the pellet, can protect the semiconductor elements and acircuit including the same from the α rays. Thus the reliability of thesemiconductor device against α rays can be improved.

(11) When the leads attached onto the circuit-forming face ornon-circuit-forming face of the pellet are provided with recesses orprotrusions in the positions thereof close to the pellet mounting partsthereof, an adhesive for bonding them to the pellet or the insulatingsheet for attachment of the pellet thereto can be prevented from flowingout to contaminate the surfaces of the bonding parts. Thus occurrence ofpoor wire-bonding can be prevented.

The invention completed by the present inventors has been specificallydescribed with reference to Examples. However, the present invention isnot limited to the above-mentioned Examples and is, needless to say,capable of various modifications within a range where they are notdeviated from the subject matter of the present invention.

For example, all Examples concern the case where the inner portions ofthe leads are directly or indirectly attached onto the main face of thepellet. However, the present invention is not limited to this case. Allor part of the inner portions of the leads may be spread near thecircuit-forming face or the main non-circuit-forming face. Moreover,although the inner portions of the leads are shown spread along anarcuate or angled path with linear sections being joined together, theinner portions of the leads may also be formed as continuous curvedsections.

Also, all Examples concern the case where only the leads on the sides ofthe pellet corresponding to the positions of the so-called short leadsare spread. However, the present invention is not limited to this case,and includes a case where leads having long inner portions are spread ina usual semiconductor device. The insulating sheet is not limited to oneof a polyimide resin, and may be one of a silicon rubber. Aheat-conductive filler such as a silicon carbide powder (SiC) may, ofcourse, be incorporated into the adhesive and/or the insulating sheet inorder to improve the heat dissipating ability.

In Examples 1 and 3, the insulating sheet may not necessarily be used.In contrast, in Example 2, an insulating sheet may be used.

The dam for preventing the adhesive of the leads from flowing out asrevealed in Example 2 is not limited to a recess, but may be aprotrusion. This kind of dam may, of course, be employed in Examples 1and 2.

The foregoing description mainly concerns the case where the inventioncompleted by the present inventors is applied to the so-called DIP (DuelIn-Line Plastic) type semiconductor device concerned with theapplication field as the background of the present invention. However,the present invention is not limited to this type of device. Thetechnique of the present invention can be effectively applied tosemiconductor devices with various forms of package structures such as aflat package structure in so far as the package is formed by sealingwith a resin.

What is claimed is:
 1. A method of producing a semiconductor devicecomprising the steps of:preparing a lead frame having a plurality ofleads each including a first end and a pellet having a circuit-formingface with a plurality of bonding pads on said circuit-forming face;bonding said plurality of leads at said first end to an insulatingsheet; after bonding said plurality of leads to said insulating sheet,bonding said insulating sheet to said circuit-forming face via anadhesive; electrically connecting one of said plurality of leads at saidfirst end to one of said bonding pads; and molding said pellet andplurality of said leads with a resin.
 2. A method of producing asemiconductor device according to claim 1, wherein said insulating sheetcomprises a polyimide resin.
 3. A method of producing a semiconductordevice according to claim 1, wherein said one of said plurality of leadsare connected to said one of bonding pads by a bonding wire.
 4. A methodof producing a semiconductor device according to claim 3, wherein saidresin comprises epoxy resin.
 5. A method of producing a semiconductordevice according to claim 1, wherein said insulating sheet is bonded tosaid circuit-forming face at a position except bonding pads.
 6. A methodof producing a semiconductor device comprising the steps of:preparing alead frame having a plurality of leads each including a first end withan insulating sheet attached to said first ends of said plurality ofleads, and a pellet having a circuit-forming face with a plurality ofbonding pads on said circuit-forming face; bonding said insulating sheetto said circuit-forming face via an adhesive; electrically connectingone of said plurality of leads at said first end to one of said bondingpads; and molding said pad and plurality of said leads with a resin. 7.A method of producing a semiconductor device according to claim 6,wherein said insulating sheet comprises a polyimide resin.
 8. A methodof producing a semiconductor device according to claim 6, wherein saidone of said plurality of leads are connected to said one of bonding padsby a bonding wire.
 9. A method of producing a semiconductor deviceaccording to claim 6, wherein said resin comprises epoxy resin.
 10. Amethod of producing a semiconductor device according to claim 6, whereinsaid insulating sheet is bonded to said circuit-forming face at aportion except bonding pads.
 11. A method of producing a semiconductordevice comprising the steps of:preparing a lead frame having a pluralityof leads each including a first end with an insulating sheet attached tosaid first ends of said plurality of leads, and a pellet having acircuit-forming face with a plurality of bonding pads on saidcircuit-forming face and a non-circuit-forming face; bonding saidinsulating sheet to said non-circuit-forming face via an adhesive;electrically connecting one of said plurality of leads to one of saidbonding pads; and molding said pellet and plurality of said leads with aresin.
 12. A method of producing a semiconductor device according toclaim 11, wherein said insulating sheet comprises a polyimide resin. 13.A method of producing a semiconductor device according to claim 11,wherein said one of said plurality of leads are connected to said one ofbonding pads by a bonding wire.
 14. A method of producing asemiconductor device according to claim 11, wherein said resin comprisesepoxy resin.